Data compiled by the group show that self-employment earnings fell by 32% – or £100 per week – between 2007 and 2014, meanwhile the numbers of self-employed jumped by more than a million people between 2002 and 2015.

Self-employment has become a more important part of the UK labour market at a time when pay for that work is in free-fall.

"The depth of the post-crisis earnings squeeze for the self-employed is striking, given that is not explained by compositional change," Adam Corlett, the report's author, said. "But, whatever the reason for these shifts, the degree of change reinforces why a full picture of the labour market must include the self-employed as well as the employed."

Real pay growth has been helped by low inflation, but typical hourly pay is still 7% below its historic peak. Wales is the region with the lowest hourly pay while London has the highest.

Resolution Foundation

The UK has a growing workforce participation rate and falling unemployment. The 18-69 year-old workforce participation level hit a record high of 75% over the last year, and has continued to rise for groups that traditionally had low activity rates.

Resolution Foundation

This includes a sharp rise in the number of people who are self-employed from 3.2 million to 4.8 million, which started around 2002.

Resolution Foundation

The numbers of self-employed people are rising, but their pay isn't. They now earn less than they did in 1994.

Resolution Foundation

Around 2/3 of this fall in earnings can be explained by so-called compositional change, which shows shifts in what kind of workers are self-employed, while the rest appears to be a fall in the number of hours worked.